The invention relates to testers and methods for testing digital circuits, and more particularly to portable computer-based testers and methods for testing digital printed circuit boards.
Modern complex digital electronic equipment is ordinarily fabricated utilizing printed circuit boards which include a large number of digital integrated circuits and other components mounted thereon. Manufacturers of such complex electronic equipment are frequently obligated to service installed units. Such servicing involves testing the electronic equipment to isolate and repair (or replace) faulty components. Consequently, it is necessary to test printed circuit boards at the site of the electronic equipment. Such "on site" testing is referred to as "field service testing", in contrast to "factory testing", the latter testing being performed by the manufacturer on printed circuit boards which must be returned to the factory for testing and repair because the field testers presently available are incapable of isolating faulty components at the site of the installed electronic equipment, and because the field service testing personnel, who ordinarily are not extensively trained in details of repairing logic circuitry of the variety of different types of printed circuit boards which may have to be tested, are incapable of making "on the spot" repairs or component replacements.
Certain of the above-described "factory testers" are capable of testing each node in the printed circuit board under test and identifying faults by comparison of each node response to a known correct response for that node. Such factory testers require that the complete data stream be stored for every node of both the printed circuit board under test and an identical "known good" board. The "factory testers" which have the above capability require large memories. In order to reduce the need for complex interface adaptor boards to "interface" between the above factory testers and various families of printed circuit boards to be tested, factory testers have utilized programmable driver/sensor circuits electrically connected to every pin of the board under test and have utilized expensive multiplexing circuitry to route data from the main processor to predetermined ones of the driver/sensor circuits in order to achieve suitably high rates of transferring input test data from the main processor to the board under test. However, such multiplexing circuitry is very expensive. In order to provide the clock signals required by many digital printed circuit boards, factory testers have utilized large serial random access memories connected in series with inputs of each of the programmable driver/sensor circuits. The data stream inputted into each input of the printed circuit board under test is initially stored in a respective one of the high speed serial random access memories. The respective data streams are then outputted as a burst of logical ones and zeros from each of the serial memories. However, the cost of providing such serial memories for this purpose is very high.
Unfortunately, because of the complexity of prior "factory testers" which have the capability of testing and isolating faults in complex digital printed circuit boards, the techniques utilized in the prior "factory testers" are unsuited for implementation in lightweight, low cost portable computer-based testers for "on site" testing of printed circuit boards of installed digital electronic equipment.
In spite of the high cost and complexity of such factory testers, in order to route power supply voltages to the appropriate power supply pins of various families of printed circuit boards, it has been necessary to use adaptor boards or relays to route programmable power supplies of the factory tester to the appropriate pins of the printed circuit board under test.
Because of the above factors, presently available factory testers cost in the range from approximately $50,000 to approximately $250,000. This cost range greatly exceeds the permissable range for a portable service tester. Unfortunately, the very high cost and large physical size of factory testers is mainly due to the inclusion therein of the features which give available factory testers their vastly improved testing and fault isolation capability over presently available portable field service testers, namely the very large memory storage requirements for storing incoming data and known correct responses for boards under test, the complex multiplexing and control circuitry associated with every input/output pin of the test, and large serial random access memories associated with each input/output pin of the tester to provide high speed inputting of data to each pin of the board under test.
Up to now, presently available portable field service testers are incapable of automatic guided probe fault isolation and are incapable of isolating faults in digital loops. Specialized adaptor boards are required to accomplish re-routing of test signals and power supply signals to the proper pins of the board under test. Further, presently available portable field service testers are incapable of generating high speed clock signals necessary to test many digital printed circuit boards, necessitating provision of clock generating circuitry on the above-mentioned adaptor boards. Further, the known portable testers (and most factory testers) have been incapable of suitably testing asynchronously operating printed circuit boards, such as microprocessor-based printed circuit boards having free running clock generating circuits. It has generally been necessary to unplug the microprocessor and/or clock generating circuitry and then attempt to test the remainder of the printed circuit board by stimulating it with signals produced by the tester.
Consequently, presently available portable field service testers must be accompanied by a variety of specialized adaptor boards to permit testing of the variety of printed circuit boards likely to be encountered in various types of electronic equipment. Due to the inability of present portable field service testers to isolate faults in digital printed circuit boards under test, it is necessary to maintain a large stock of "spare" boards which are utilized to temporarily replace printed circuit boards found to be defective in the electronic equipment, and to ship the defective boards back to the factory for thorough testing using a factory tester. The defects, once isolated at the factory, are then repaired, shipped back to the site of the installed electronic equipment and "plugged in", and the temporary "spare" board is returned to the inventory of "spares". The high overhead costs and inconvenience associated with maintaining an inventory of "spare" printed circuit boards as well as a variety of adaptor boards presently adds greatly to the inconvenience and cost of field service testing.
A number of U.S. patents have been found which are believed relevant. U.S. Pat. No. 3,739,349 discloses a programmable driver/sensor circuit. U.S. Pat. No. 3,922,537 discloses an automatic tester having programmable multiplexing apparatus to electrically connect different measurement and stimulus devices to each pin of the board under test. The latter approach is used by most factory testers, and is too bulky and too expensive for utilization in portable field service testers.
The forefront of the art in the area of portable service processors is believed to be generally indicated by U.S. Pat. Nos. 3,976,864 and 3,924,181 and is also believed to be indicated by the following articles: "Signature Analysis: A New Digital Field Service Method", by Robert A. Frohwerk, pages 2-8 Hewlett-Packard Journal, May, 1977; "Easy-to-Use Signature Analyzer Accurately Trouble-Shoots Complex Logic Circuits" by Anthony Y. Chan, pages 9-14, Ibid., and "Signature Analysis-Concepts, Examples and Guide Lines" by Hanns J. Nadig, pages 15-21, Ibid.
Other references found in a search pertaining to the present disclosure include the following U.S. Pat. Nos.: 3,274,529; 3,325,766; 3,336,434; 3,409,828; 3,549,996; 3,646,438; 3,657,527; 3,676,777; 3,739,349; and 3,922,537.
There is clearly a great need at the present time for a low cost portable field tester capable of permitting ordinary service personnel to quickly "field test" individual printed circuit boards in installed electronic equipment and isolate faults at the component level so that faulty components may be quickly identified and replaced at the installation site of the electronic equipment. It is necessary that such a low cost portable field tester have testing and fault isolation power approaching that of factory testers, which are presently much larger and more expensive than is acceptable for a portable field service tester.